( 321 ) 
that which is brought about the apex and the margin of the leaf 
and on the leaf-surface as a result of the action of glands. 
In many cases the glands are originally mucilage-glands (Colle- 
teren, Keulenzotten. Trichomzotten) which secrete resin or balsam in 
the bud, as proved to be the case in Kerria, Sarnbucus, Corylus, 
Ulmus, ’ Sy ring a > Forsythia , but in other plants they are from the 
beginning real water-glands: Philadelphus , Deutzia,, Hydrangea,, 
Weigelia, etc. 
Physics. — "On the theory of the Zef.man-«//«c< in a direction 
inclined to the lines of force.” By Prof. H. A. Lorentz. 
(Communicated in the meeting of June 26, 1909) 
$ 1. Certain phenomena observed by Hale in sun-spot spectra 
have induced me to work out the theory of the ZEEMAN-effect on 
the assumption that the direction of observation is oblique to the 
lines of force, a problem that has already been treated by Voigt ),. 
but in which some details remained to be examined. 
Our subject will be the ‘‘inverse” effect, to which the direct one 
is intimately related, and we shall start from the fundamental equa¬ 
tions in the form I have given them in a recent article in the 
“Mathematische Encyklopadie” 1 2 ), supposing the magnetic field to.be 
homogeneous and parallel to the axis of z. 
We shall assume that the particles of the body through which 
the light is propagated, unless they be magnetically isotropic (i. e. 
of such a structure that a rotation of a particle in the field has no 
influence on the frequency of its free vibrations) are turned by the 
magnetic force in such a manner that a certain “axis proper to each 
particle takes the direction of the field. We shall further imagine 
that each particle contains a certain number of electrons forming by 
their arrangement some definite and regular configuration, and capable 
of vibrating about their positions of equilibrium under the joint, 
influence of “quasi-elastic” forces, of resistances and of the action 
exerted by the external field. Though, on account of the complexity 
of its structure, the mode of motion of a particle may be far from 
simple, we can easily treat it mathematically in a general way. This 
is due to the circumstance that, under certain simplifying restrictions, 
1 ) W. Voigt, Weiteres zur Theorie der magneto-optischen Wirkungen, Ann. 
Phys. I (1900), p. 389. 
2 ) H. A. Lorentz, Theorie der magneto-optischen Phanomene, Encyklopadie 
d. math. Wiss. V 22, p. 199. 
